ANATOMY OF TISSUES. 53 
dividing wall is either straight or curved in various ways by 
which the shape of the mother cell is modified. This latter 
then divides into two equal cells, a wall forming through it 
which afterwards splits along its central portion, making an 
opening to the space below. 
The guard cells at first lie in the same plane with the 
remaining epidermal cells, but as they develop their position 
may be changed so that they are either raised above or sunk 
below the other cells. In the latter case there is a little depres- 
FIG. 25. 
A cross-section through the upper epidermis and the neighboring tissue of the leaf of 
Pinus Laricio. O epidermis, h hypoderma, g green parenchyma, S guard cells 
with chlorophyll, a air space, v outer court of stoma. B surface appearance of the 
stoma, ss guard cells, S opening of the stoma. x 390. —( Wiesner.) 
sion of the epidermis over the guard cells which is called the 
outer court. This arrangement has to do with the control of 
exchange of gases, the sunken stoma being the most common. 
This is farther regulated by the ability of the guard cells to 
open or close the aperture between them. The mechanics of 
this action has been studied by von Mohl in 56, by Schwendener 
in ’81, and by others at different times, and is found to 
depend principally on two factors, the pressure or turgor, 
and the uneven thickening of wall by which a portion is left 
thin acting somewhat like a hinge. They are generally closed 
at night and open in the daytime, and with the increasing 
turgor of the guard cells the opening widens; with the decrease 
